The invention concerns a method for the open-loop and closed-loop control of an internal combustion engine with an independent A-side common rail system and an independent B-side common rail system, in which in normal operating mode, the rail pressure is automatically controlled in each common rail system by a suction throttle on the low-pressure side as a first pressure regulator in a closed-loop rail pressure control system, and at the same time, the rail pressure is acted upon with a rail pressure disturbance variable by means of a pressure control valve on the high-pressure side as a second pressure regulator by virtue of the fact that a pressure control valve volume flow is redirected from the rail into a fuel tank by the pressure control valve on the high-pressure side.
In an internal combustion engine with a common rail system, the quality of combustion is critically determined by the pressure level in the rail. Therefore, in order to stay within legally prescribed emission limits, the rail pressure is automatically controlled. A closed-loop rail pressure control system typically comprises a comparison point for determining a control deviation, a pressure controller for computing a control signal, the controlled system, and a software filter in the feedback path for computing the actual rail pressure from the raw values of the rail pressure. The control deviation in turn is computed as the difference between the set rail pressure and the actual rail pressure. The controlled system comprises the pressure regulator, the rail, and the injectors for injecting the fuel into the combustion chambers of the internal combustion engine. For example, DE 103 30 466 B3 describes a common rail system of this type, in which the pressure controller acts on a suction throttle arranged on the low-pressure side by means of a control signal. The suction throttle in turn sets the admission cross section to the high-pressure pump and thus the volume of fuel delivered.
The unprepublished application DE 10 2009 031 527.6 also describes a common rail system with automatic control of the rail pressure by means of a suction throttle on the low-pressure side as a first pressure regulator. This automatic pressure control in the common rail system is supplemented by a pressure control valve on the high-pressure side as a second pressure regulator, by which pressure control valve volume flow is redirected from the rail into the fuel tank. A constant leakage of, for example, 2 liters/minute is reproduced in the low-load range by means of activation of the pressure control valve. Under normal operating conditions, on the other hand, no fuel is redirected from the rail. The pressure control valve volume flow is determined on the basis of a set volume flow with a static and a dynamic component. In the computation of the dynamic component and the computation of the control signal for the closed-loop rail pressure control system, the actual rail pressure is a critical input variable. Therefore, a defective rail pressure sensor or an error in the signal acquisition of the rail pressure results in a false actual rail pressure and causes faulty activation of both the suction throttle as the first pressure regulator and the pressure control valve as the second pressure regulator. The cited document fails to provide any fault safeguard in the event of failure of the rail pressure sensor.
DE 10 2006 040 441 B3 describes a common rail system with closed-loop pressure control, in which a passive pressure control valve is provided as a protective measure against excessively high rail pressure, for example, after a cable break in the power supply to the suction throttle. If the rail pressure rises above a critical value, for example, 2400 bars, the pressure control valve opens. The fuel is then redirected from the rail to the fuel tank through the open pressure control valve. With the pressure control valve open, a pressure level develops in the rail which depends on the injection quantity and the engine speed. Under idling conditions, this pressure level is about 900 bars, but under a full load, it is about 700 bars.
DE 10 2007 034 317 A1 describes an internal combustion engine with an independent A-side common rail system and an independent B-side common rail system, which are identical in structure. The two common rail systems are hydraulically decoupled from each other and therefore allow independent closed-loop control of the A-side and B-side rail pressure. Pressure fluctuations in the rails are reduced by the separate closed-loop control. Correct closed-loop rail pressure control requires properly operating rail pressure sensors. The failure of one rail pressure sensor or both rail pressure sensors in the specified system results in an undefined state of closed-loop pressure control and can produce a critical state of the internal combustion engine, since the cited document fails to indicate any fault safeguards.